Liquids, mixtures, solutions and reacting mixtures are often characterized using optical techniques such as photometry, spectrophotometry, fluorometry, or spectrofluorometry.
In particular, sampling techniques used in UV-Visible spectrophotometry can include using cuvettes configured with one or more optical windows and a fixed optical path length so as to hold a sample in a semi-closed way. Such a cuvette-based vessel method is typically incorporated by pipetting a sample into such a vessel that has either a 10 mm or a 2 mm path length. The method itself, is limited for most biological samples since cuvettes most often require a mL sample, which is often discarded after measurements. Large sample volume and loss can thus be problematic for valuable biological samples which are often of limited quantities. Moreover, directed samples into such cuvettes can produce air-interface bubble interfaces in the optical measurement path, which can produce measurement errors. Additionally, a path-length of 2 mm or 10 mm limits the sample concentration that may be measured to 1000 ng/ml for a DNA/RNA sample due to the limited dynamic range of absorbance of most spectrophotometers.
To overcome the aforementioned difficulties of handling biological samples of limited quantities, and/or of which requires dilution and/or which has contamination issues, other techniques, such as those similarly disclosed in U.S. Pat. Nos. 6,809,826 and 6,628,382, which are incorporated herein by reference in its entirety, have been developed to enable a user to investigate path lengths in the range from about 0.2 to 2 mm and generate absorbance values that can be easily corrected to using a cuvette-based techniques. According to the teachings in the aforementioned patents, a smaller sample volume, which is incapable of being investigated in a cuvette-based apparatus, is held between two opposing substantially parallel surfaces by interfacial tension and one surface is controllably moved toward and/or away from the other. To provide and transmit light through the droplet for measurement, and to collect light for measurement, at least one of the surfaces may have a portion of optical measurement quality. This may be accomplished by providing at least a portion of at least one of the surfaces as a polished end of an optical fiber, each such optical fiber preferably finished flush with the surrounding surface portion. Conveniently, such surrounding surface portion may comprise the surface of an end of a standard fiber optic connector or other fiber holder.
However, while the aforementioned interfacial tension techniques have beneficial aspects over cuvette-based methods, a need urgently exists for an integrated spectroscopic apparatus that is configured to interrogate small sample volumes while additionally capable of handling larger sample volumes configured in vessels having measurement lengths of up to about 25 mm, more often of up to about 10 mm. The reasoning is that such an integrated apparatus and corresponding methods thereof enable a user to cross-calibrate measurements within the instrument itself in addition to cross-calibrating and interfacing with other existing conventional instruments and methods currently in the industry. Accordingly, the present invention is directed to such a need.